The invention relates to improvements in a liquid encapsulation Czockralaski method (LEC method) for growing a single semiconductor crystal by pulling up a seed crystal covered with a liquid encapsulant such as B.sub.2 O.sub.3 out of a semiconductor compound melt in a heated crucible. The invention also relates to an improved apparatus for crystal growth used in carrying out the LEC method.
Semiconductor compounds produced by the LEC method include compounds of elements of Group III and Group V of the Periodic Table for example, GaAs, GaP, InP, InAs, GaSb, etc. and compounds of elements of Group IV and Group VI of the Periodic Table for example, PbTe, PbSe, SnTe, etc.
Conventional apparatus for growing a single crystal based on a LEC method generally have a single cylindrical crucible heater with a relatively large uniform thickness of heating element structure around a quartz crucible supported by a carbon support.
Another LEC apparatus for growing a single crystal is disclosed in Japanese Patent Publication No. 39787/1977 wherein the inner diameter and outer diameter of a single cylindrical quartz crucible heater vary along the axial line of the crucible adjacent the walls of the crucible.
Another LEC apparatus has two cylindrical crucible heaters where a portion of one heater is positioned near the top of the crucible and a portion of the second heater is positioned near the bottom of the crucible. Such an apparatus is disclosed in Japanese Patent Laying Open No. 11897/1982 wherein a quartz crucible is supported by a carbon support around which the two heaters are installed. The two heaters can be independently moved upwardly or downwardly. The temperature distribution in the crucible is changed by moving the crucible and its carbon support and by moving the two independent heaters upwardly or downwardly.
The above described apparatuses for carrying out an LEC method for producing a single crystal semiconductor are directed to provide a temperature environment in the crucible suitable for crystal growth in the crystal growing region of the crucible. However, a significant problem has not been addressed. In practice, the crystal pulled up from a semiconductor compound melt is cooled rapidly with non-uniform temperature distribution at a cooling zone above the crystal growing region above the crucible.
The rapid and non-uniform cooling of a single crystal semiconductor brings about the following undesirable results.
1. When the single crystal that is pulled up is cooled abruptly and irregularly by convection of highly-pressurized gas at the cooling zone above the crucible, strong thermal stress is generated in the crystal by the non-uniform distribution of temperature. As a result, many dislocations, lineages, and other lattice defects result within the crystal.
2. Even if the temperature gradient between (a) the crystal growing region corresponding to the vicinity of liquid-solid interface and (b) the space in the liquid encapsulant is reduced in order to avoid rapid cooling in the interface region, the generation of thermal stress is not adequately avoided. Large amounts of heat are transmitted vertically in the grown crystal and from the surface of the upper portion of the crystal. This occurs because the upper portion of the crystal is still exposed to a strong convection current of pressurized gas. This mode of heat dissipation causes radial temperature differences between the surface and the center of the crystal in horizontal planes normal to the vertical axis of the crystal. Thus, the grown crystal is fragile and vulnerable. The crystal is susceptible to cracking during cooling. In such a case, when the cooled crystal ingot is sliced into thin wafers, both the ingot and the wafers are apt to crack.
Accordingly, it is an object of the present invention to provide a liquid encapsulation Czockralski method and apparatus for growing single semiconductor crystals wherein control of the thermal environment of the cooling zone above the crystal growing crucible is provided.